pH meter theory

[jdl]

I understand that a potential difference is created between the inside and outside of the glass membrane, and that the potential difference is proportional to the hydrogen ion concentration. But how exactly does that potential difference affect the half cell reaction that occurs at the internal Ag/AgCl reference electrode? Thanks in advance.

[ewmon]

If this is homework help, it belongs in the Homework forum.

 

To answer your question: It doesn't.

[jdl]

If this is homework help, it belongs in the Homework forum.

 

To answer your question: It doesn't.

 

This isn't for homework, just for me to understand the mechanism behind a pH meter and other forms of ion-selective electrodes.

 

So if the external reference electrode potential is constant, and if the internal reference electrode potential isn't affected by the potential difference that develops at the glass membrane (as a result of a difference in hydrogen ion concentration/activity outside the and inside the glass membrane) as ewmon says, then how is the membrane potential that inevitably develops at the glass membrane detected, and consequently alters the potential difference (voltage) between the two electrodes?

[ewmon]

A spot on the glass membrane allows only protons through, which makes the sensing electrode ion-sensitive, and the protons enter the solution in the bulb until equilibrium occurs in the concentration of these ions across the membrane. To complete the circuit, the reference electrode is in a solution in contact with the (outside) test solution, and to avoid contamination, the electrode is designed for it to slowly leak into the test solution, but this junction is not ion sensitive. This sets up a kind of battery powered only by the ions to which the electrode has been made sensitive.

Edited February 11, 2012 by ewmon